This invention relates to the field of electrical circuit condition monitoring, and more particularly to a monitoring means for determining the existence of high and low resistance conditions in a circuit external to the monitoring means.
In certain military airborne electronic, or avionics, systems predetermined high and low resistance conditions are indicative of operating modes, states of readiness, or the like, which are required to be known. In the past, such avionics systems having this requirement have utilized a monitor circuit which has as its principal element a +28 VDC, electromechanical relay and relies on the difference between pick-up current and the drop-out current of the relay to control alternative energization of utilization means such as system control logic or first and second indicator lamps to indicate existence of predetermined high or low resistance conditions (R.sub.H and R.sub.L) in the circuit being monitored. In the actual avionics application the interface conditions are specified and the monitor circuit is designed to meet these requirements. A range of resistance values is specified for both R.sub.L and R.sub.H. Additional specifications include: a minimum and maximum value for I.sub.L the permissible current flow through R.sub.L, maximum and minimum values of open circuit voltage, and minimum isolation resistance between the monitor circuitry and ofher interface functions. In addition, the monitor circuit must meet the applicable environmental requirements including extended temperature range operation, high humidity, and extremes of vibration and shock. High reliability and fail-safe operation are also required of the monitor circuit.
Electro-mechanical relay parameters exhibit wide tolerances, and vary considerably with such environmental factors as temperature and vibration. Both pick-up and drop-out current vary over a wide range. The relay coil, itself, is wound from copper magnet wire. The resistive component of coil impedance, therefore, exhibits a large positive temperature coefficient. The coil resistance increases by 40 percent at +125.degree. C. as referred to the +25.degree. C. value. The inductive portion of the coil impedance is highly non-linear. The exact value of inductance depends on the degree of relay closure since the width of the magnetic air-gap varies correspondingly.
It has further been determined that for the relay type of monitor circuit to be implementable the ratio of the maximum pick-up current to the minimum drop-out current must be less than the ratio of R.sub.H (minimum) to R.sub.L (maximum). Recent advances in certain avionics systems have resulted in a requirement for monitoring resistance conditions where the ratio of R.sub.H to R.sub.L is considerably lower than the ratio of pick-up to drop-out currents, and the known relay type of monitoring circuit cannot be implemented.
An additional requirement of newer avionics systems is the need for the resistance monitoring circuit to be operable at low frequency AC. Under conditions of continuous AC operation electro-mechanical relay lifetime limitations present reliability and/or maintenance problems.